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1. Interference from maternal antibodies
   If young pets receive a vaccine before the maternal antibodies it acquired from its mother are gone, the vaccine will be blocked and no immunity will develop
2. Unhealthy pets
   A sick animal may have difficulty producing antibodies against a vaccine because its immune system is suppressed
3. Improper handling of vaccines
   If vaccines are shipped or stored in an area that is too warm, they can lose their potency and effectiveness